Chair base



FIG. 1 is a perspective view of a chair base showing my design;

FIG. 2 is a top plan view thereof;

FIG. 3 is a bottom plan view thereof;

FIG. 4 is a front elevational view thereof;

FIG. 5 is a side elevational view thereof; and

FIG. 6 is a rear elevational view thereof. 

The ornamental design for a chair base, as shown and described. 